Conventionally, when video data are exchanged between television broadcasting stations or when video data are copied by, a plurality of videotape recorders (VTR devices), the video data encoded in MPEG (Moving picture experts group) are first decoded and then encoded again. This is why an encoder and a decoder need to be serially connected in tandem.
And such tandem systems repeat encoding and decoding, and the quality of video data deteriorates accordingly. To prevent this, so-called “back search” can be applied.
Here, “back search” is to calculate a quantization step of a minimum value as an appropriate quantization step based on the following characteristic: if a quantization step used in the previous encoding process or a quantization step that is a multiple of it is used, the sum of residues of DCT (discrete cosine transfer) coefficients becomes minimum.
A video data compression device equipped with the back search function is disclosed in Patent Document 1, for example: in order to prevent the deterioration of the video quality, which might occur due to the tandem connection of encoders and decoders and a repeat of encoding and decoding, the device uses the back search function to reproduce a quantization step of the previous encoding process, and uses the same quantization step and GOP (Group of pictures) phase as the previous one did to encode input video data again.
On the other hand, instead of MPEG, AVC (Advanced Video Coding) is becoming popular: it is used for low-speed and low-quality video transmission, such as a television telephone of a cell phone, and sizeable, high-quality video transmission, such as high-definition broadcast. AVC is a combination of algorithms, including motion compensation, interframe prediction, DCP, and entropy coding. It can offer the same quality as MPEG with half the amount of data.
And, especially, for AVC intra images (referred to as “AVC Intra,” hereinafter), a quantization parameter QP is appropriately determined based on a target bit rate for quantization, but this quantization parameter QP is not necessarily the same as the one used in the previous encoding process. Using a different quantization parameter QP from the previous one may cause a deformation due to the rounding of the subsequent quantization processes; dubbing video many times hurts the quality of video data.
Patent Document 1: Japanese Patent Publication No. H10-174098
However, even if AVC Intra uses the same detection technique as that of MPEG to solve the above-mentioned dubbing problem, this may not work: dubbing causes the quality of video data to deteriorate because their encoding systems are different and the accuracy of back search therefore decreases.